System for streamer electrical resistivity survey and method for analysis of underground structure below a riverbed

ABSTRACT

A system for measuring electrical resistivity survey checks a border of bedrock or a thickness of a sedimentary layer in a riverbed of a river or lake within a short time. A method for analysis of an underground structure of a riverbed using the same is also provided. The system for streamer electric resistivity survey using a survey boat comprises a streamer cable connected to the survey boat and having a plurality of electrodes attached thereto; a multi-channel resistivity meter loaded on the survey boat to measure electric resistivity from the plurality of electrodes; a first RTK GPS (Real Time Kinematic Global Positioning System) loaded on the survey boat to measure a position of the survey boat in real time; and a second RTK GPS installed to a tail of the streamer cable to measure a position of the tail in real time.

TECHNICAL FIELD

The present invention relates to a system for streamer electricalresistivity survey and a method for analysis of an underground structurebelow a riverbed, and more particularly to a system for measuringelectrical resistivity survey on a water surface to check a border ofbedrock or a thickness of a sedimentary layer in a riverbed of a riveror lake, a system for electrical resistivity survey, which analyzes anunderground structure by modeling the measured electrical resistivity,and a method for analysis of an underground structure of a riverbedusing the same.

BACKGROUND ART

Checking a border of bedrock or a thickness of a sedimentary layer in ariverbed of a river or lake is essential to calculating a sediment yieldfor riverbed construction, ensuring the stability of bridge design, andoptimizing excavation for canal design.

In order to check a border of bedrock, drilling is generally used. Thedrilling may give most exact data for borders of bedrock, sediment andweathered layer, but this method just gives depth information limited tothe drilling spot and takes much time and cost for the drilling work.

As another method, GRP (Ground Penetrating Radar) survey or shallowsurvey may be applied, but they have the following problems. First, theGRP survey has a limitation in the depth of investigation according toelectric conductivity of a river or lake, and particularly penetrationof radar wave is limited in case a soil sedimentary layer has a lowelectric conductivity. Also, the shallow survey needs development of asuitable transmitting source, so it is also required to develop areceiver sensor accordingly.

Due to the above problems, the present invention is directed to applyingan electric resistivity survey at water-covered area. Generally, theelectric resistivity survey is conducted in a way of flowing a certaincurrent to a conductor and then calculating a potential differencebetween predetermined two points to calculate resistivity between twopoints. That is to say, the electric resistivity survey is a geophysicalmethod for intentionally flowing current into the ground to measure apotential difference and then calculating electric property distributionof an underground media to check a geological structure or anomaly,thereby imaging a geological structure that is complicatedly formed dueto irregular media.

DISCLOSURE Technical Problem

The above electric resistivity survey is generally used along an earthsurface to check a geological structure under the ground. However, incase the electric resistivity survey is used at water-covered area,electric resistivity may be differently measured according to the changeof electrode positions on the water, and this problem should be solved.Thus, for more exact analysis, electric resistivity data for the wateraccording to a measured position is additionally needed such that onlythe underground structure is analyzed except for the undesired waterportion.

The present invention is designed to solve the problems of the priorart, and therefore it is an object of the present invention to provide asystem for electric resistivity survey, which may check a thickness ofsedimentary layer and a border of bedrock below a river or lake withhigher accuracy in a short time by measuring electric conductivity ofwater in the river or lake as well as electric resistivity according tothe change of electrode positions on the water in real time; and amethod for analysis of an underground structure below a riverbed.

Technical Solution

In order to accomplish the above object, the present invention providesa system for streamer electric resistivity survey, which surveys anunderground structure below a riverbed using a survey boat, the systemcomprising: a streamer cable connected to the survey boat and having aplurality of electrodes attached thereto; a multi-channel resistivitymeter loaded on the survey boat to measure electric resistivity from theplurality of electrodes attached to the streamer cable; a first RTK GPS(Real Time Kinematic Global Positioning System) loaded on the surveyboat to measure a position of the survey boat in real time; and a secondRTK GPS installed to a tail of the streamer cable to measure a positionof the tail of the streamer cable in real time.

The survey boat may further include a conductivity meter mounted to alower portion of the survey boat to measure an electric conductivity ofwater; and a side scan sonar mounted to the lower portion of the surveyboat to measure a water depth.

Using the present invention, it is possible to provide a new surveysystem capable of analyzing an underground structure below a riverbedwithin a short time by measuring electric conductivity of a riverbed andlimit condition data (e.g., water depth, water temperature and electricconductivity of water) of the river water according to changingpositions using position information obtained by RTK GPS.

In the present invention as mentioned above, the streamer cablepreferably has a tail buoy connected to the tail thereof such that thestreamer cable is kept in a linear state, and buoys are preferablyinstalled to the streamer cable among the plurality of electrodes suchthat the electrodes are floating in contact with water.

In addition, the second RTK GPS preferably has a radio modem by whichposition information of the tail of the streamer cable may betransmitted to the survey boat, and the transmitted position informationof the tail of the streamer cable is calculated with the positioninformation of the survey boat measured by the first RTK GPS to providea precise. measurement position.

In addition, the system for streamer electric resistivity surveyaccording to the present invention may further include an operatingdevice to which measurement data is input, the operating device imagingan underground structure below a riverbed by means of electricresistivity inversion.

The operating device executes: generating GIS (Geographic InformationSystem) data from the electric conductivity measured by the conductivitymeter and the water depth measured by the side scan sonar using theposition data measured by the first RTK GPS; calculating precise GPSdata of the measurement position from the position data measured by thefirst RTK GPS and the position data measured by the second RTK GPS; andconducting two-dimensional inversion to the electric resistivity datameasured by the multi-channel resistivity meter based on the GPS data ofthe measurement position, and imaging an underground structure below ariverbed using the inversion result.

At this time, when the operating device conducts two-dimensionalinversion to the measured electric resistivity data, the water depthdata and the electric conductivity data of water are input thereto aslimit conditions.

By using the above operating device, it is possible to image anunderground structure below a riverbed, and accordingly it is possibleto easily check a thickness of sedimentary layer and a border of bedrockin the riverbed.

In addition, in order to accomplish the above object, the presentinvention provides a method for analysis of an underground below ariverbed using the measured data at an operating device that includes amemory, a data operator, an underground structure analyzer, and acontroller, the method comprising: (c) the data operator readingposition data of a survey ship and position data of a tail of a streamercable from the memory to generate precise measurement position data; (d)the underground structure analyzer conducting two-dimensional inversionto the measured electric resistivity data according to the precisemeasurement position data; and (e) the underground structure analyzerimaging an underground structure below a riverbed using the inversionresult, and, in the step (d), a water depth and an electric conductivityof water are input as limit conditions.

In another aspect of the present invention, there is also provided amethod for analysis of an underground below a riverbed using themeasured data at an operating device that includes a memory, a dataoperator, an underground structure analyzer, and a controller, themethod comprising: (a) the data operator reading electric conductivitydata of water and position data of a survey ship from the memory togenerate an electric conductivity map of water according to position;(b) the data operator reading water depth data and the position data ofthe survey ship from the memory to generate a water depth map accordingto position; (c) the data operator reading the position data of thesurvey ship and position data of a tail of a streamer cable to generateprecise measurement position data; (d) the underground structureanalyzer conducting two-dimensional inversion to the measured electricresistivity data according to the precise measurement position data; and(e) the underground structure analyzer imaging an underground structurebelow a riverbed using the inversion result.

In the step (d), the electric conductivity data of water according toposition obtained in the step (a) and the water depth data according toposition obtained in the step (b) are input as limit conditions.

Using the above method, data related to water level and electricconductivity of water according to position are inserted as limitconditions during the inversion, so the inversion may be conducted onlyfor the underground structure below a riverbed, which allows to givevery high accuracy rather than the inversion including river water.

Advantageous Effects

As described above, in analyzing an underground structure below ariverbed based on a measured electric resistivity, the present inventionallows to rapidly measure data of river water and electric resistivityof changing positions at the same time using real-time positioninformation obtained by RTK GPS, and also to give exact analysis resultsby decreased error obtained by inputting electric conductivity and waterdepth at each position as limit conditions during inversion.

That is to say, the present invention makes it possible to check athickness of sedimentary layer and a border of bedrock below a riverbedof a river or lake exactly within a short time.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a system for streamer electricresistivity survey according to one embodiment of the present invention.

FIG. 2 is a schematic view showing a device applied to the system forstreamer electric resistivity survey according to one embodiment of thepresent invention.

FIG. 3 is an algorithm for analyzing an underground structure below ariverbed using the system for streamer electric resistivity surveyaccording to one embodiment of the present invention.

<Reference Numerals of Essential Parts in the Drawings> 100: survey boat110: streamer cable 112: electrode 114: buoy 116: tail buoy 120: secondRTK GPS 130: conductivity meter 140: side scan sonar 150: first RTK GPS160: multi-channel resistivity meter 200: operating device 210: memory220: data operator 230: underground structure analyzer 240: controller

BEST MODE

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

The present invention is not limited to the following embodiment, but itmay be implemented in other ways. Rather, the embodiment introducedbelow is just for better understanding of the spirit of invention tothose having ordinary skill in the art, such that the followingdisclosure would becomes more thorough and perfected.

FIG. 1 is a block diagram showing a system for streamer electricresistivity survey according to one embodiment of the present invention,and FIG. 2 shows a device for the system.

The system for streamer electric resistivity survey according to thepresent invention includes a survey boat 100 on which various measuringequipment is loaded, and a streamer cable 110 to which a plurality ofelectrodes are attached.

A multi-channel resistivity meter 160 and a first RTK GPS (Real TimeKinematic Global Positioning System) 150 are loaded on the survey boat100, and a conductivity meter 130 and a side scan sonar 140 may beinstalled to a lower portion of the survey boat 100.

The multi-channel resistivity meter 160 measures electric resistivity,transmitted from the plurality of electrodes attached to the streamercable 110, together in multi channels, and the first RTK GPS 150measures a position of the survey boat in real time with centimeterlevel.

The conductivity meter 130 mounted to the lower portion of the surveyboat inside water measures electric conductivity of water, and the sidescan sonar 140 measures a water depth of a river or lake usingultrasonic waves. Here, the measured water depth and electricconductivity are used as limit conditions during inversion.

Generally, in case an underground structure is checked using inverseoperation after electric resistivity survey, the entire lower area ofelectrodes is subject to the inversion. In case the survey is conductedusing electrodes floating on a river as in the present invention, theriver and the entire geological layer below it are all subjected to theinversion. However, in case the river and the entire geological layerbelow it are all subjected to the inverse operation, there occur seriouserrors in analysis of an targeted underground structure below a riverbedof a river. Thus, in the inversion mode, depth and electric resistivity(a reciprocal of electric conductivity) of water are put as fixedvalues, and inversion is conducted only to a geological layer in theriver using the electric resistivity measurement result. In this case,better accuracy is ensured rather than the case of conducting inversionincluding the water.

For this purpose, there are needed data for depth and electricconductivity (a reciprocal of electric resistivity) of the water to beused as limit conditions during the inversion. In the present invention,the conductivity meter 130 and the side scan sonar 140 are mounted tothe lower portion of the survey boat 100, so water depth and electricconductivity of water to be used as limit conditions for inversion aremeasured together, thereby ensuring rapid and accurate analysis of anunderground structure below a riverbed.

Such electric conductivity and water depth data of the river may bereplaced with data separately measured regardless of the survey systemof the present invention. In this case, the conductivity meter and theside scan sonar may not be mounted to the survey system of the presentinvention.

The plurality of metal electrodes 112 for measuring electric resistivityare attached to the streamer cable 110 connected to the survey boat 100,and small buoys 114 are installed among the plurality of electrodes suchthat the electrodes 112 are floating in contact with water. A tail buoy116 is connected to a tail of the streamer cable 110 such that thestreamer cable is kept in a linear state, and a second RTK GPS 120 isinstalled to the tail buoy 116 to measure an accurate position of thetail of the streamer cable in real time. In addition, the second RTK GPS120 is provided with a radio modem by which position information istransmitted, to the survey boat 100, and the position information isrecorded together with the position of the survey boat 100, measured bythe first RTK GPS 150.

In addition, the system for streamer electric resistivity surveyaccording to the present invention may further include an operatingdevice 200 to which the measurement data is input. The operating device200 images an underground structure below a riverbed by means ofelectric resistivity inversion. This operating device 200 may bedirectly loaded to the survey boat 100 to show an image of theunderground structure below a riverbed in real time according to thesurvey data, and the operating device 200 may also be not loaded on thesurvey boat 100 such that the underground structure below a riverbed maybe imaged separately on the ground based on the stored measurement data.

The operating device 200 may include a memory 210, a data operator 220,an underground structure analyzer 230 and a controller 240. The memory210 stores various measurement data. The data operator 220 reconfiguresthe measurement data into position data that may be applied to analysisof an underground structure. The underground structure analyzer 230conducts two-dimensional inversion based on the electric resistivitydata and images an underground structure based on analysis results of afinal underground electric resistivity model. The controller 240controls the above operating process.

The operating device 200 executes the following steps using an algorithmshown in FIG. 3 in order to image an underground structure below ariverbed.

First, the data operator generates GIS (Geographic Information System)data from the electric conductivity data of water measured by theconductivity meter 130 and the water depth data measured by the sidescan sonar 140 by using the GPS data. That is to say, the data operator220 executes (a) reading electric conductivity data of water andposition data of the survey ship from the memory to generate an electricconductivity map of water according to position (Step S312), and readingwater depth data and position data of the survey ship from the memory210 to generate a water depth map according to position (Step S314).

Then, GPS data of a precise measurement position is calculated using theposition data measured by the first RTK GPS 150 and the position datameasured by the second RTK GPS 120. That is to say, the data operator220 executes (c) reading position data of the survey ship and positiondata of the tail of the streamer cable to generate precise measurementposition data (Step S316). In this way, precise position data formeasurement position of each electrode may be calculated based on theposition data of the first RTK GPS and the position data of the secondRTK GPS.

Then, two-dimensional inversion is conducted to the electric resistivitydata, measured by the multi-channel resistivity meter 160, based on theGPS data of the measurement position. That is to say, the undergroundstructure analyzer 230 executes (d) conducting two-dimensional inversionto the measured electric resistivity data according to the preciseposition data (Step S320), and (e) imaging an underground structurebelow a riverbed using the inversion result (Step S322).

At this time, in the step (d), the electric conductivity data of wateraccording to position obtained in the step (a) and the water depth dataaccording to position obtained in the step (b) are input as limitconditions.

Using the inversion step S320, analysis results of a final undergroundelectric resistivity model may be derived, an underground structurebelow a riverbed may be imaged using the analysis results, and athickness of sedimentary layer and a border of bedrock below a riverbedmay be easily checked by analyzing the image. In particular, in theinverse operation step S320, since data for water depth and electricconductivity of water according to position are inserted as limitconditions, it is possible that the inversion is conducted only for anunderground structure below a riverbed, which ensures much higheraccuracy rather than inversion including the river water.

In the analysis of electric resistivity survey data, the inversion isconducted in a way of calculating a theoretical potential or an apparentresistivity of each measurement point for an assumed undergroundelectric resistivity model, correcting the underground electricresistivity model such that the theoretical value approximates to themeasured survey data, and repeatedly conducting the same. If adifference between the theoretical value and the measurement value issufficiently decreased, it may be considered as being converged, and atthis time an underground structure is imaged based on the analysisresult of the final underground electric resistivity model. Suchinversion and imaging processes are conducted using an analysis softwareconfigured based on the above algorithm.

The electric conductivity of water and the water depth inserted duringthe inversion step S320 may be measured separately from the surveysystem of the present invention, and other stored data in GIS format maybe used. In this case, among the above steps, the step (a) (S312) andthe step (b) (S314) may be not executed.

Mode for Invention

Hereinafter, operations of the system for streamer electric resistivitysurvey, configured as above according to the present invention will beexplained. The survey boat 100 moves to a target survey spot, floats thestreamer cable 110 on the water, and applies current to the electrodes112 attached to the streamer cable 110 in contact with water. Thiscurrent is applied into the water through the electrodes 112, and thiscurrent is flowed to other electrodes 112 through current paths.

At this time, an isoelectric line with a potential obtainedperpendicular to the current paths is obtained, and this isoelectricline is extended to the water surface, so a difference of isoelectriclines among the electrodes 112, namely a potential difference, ismeasured, and then the data is inversely input to the multi-channelresistivity meter 160. Thus, using the current amount flowed to theelectrodes 112 and the measured potential difference, it is possible tofind an accurate real electric resistivity of a homogeneous undergroundmedia.

In addition, if there exists any material with different electricresistivity, the current is flowed more to a material with a lowerelectric resistivity, thereby causing a change to the isoelectric lineperpendicular to the path, which also gives an influence to thepotential difference measured on the water surface. Thus, from theabove, it is possible to obtain an apparent electric resistivity havinginformation related to an electrically anomaly of the underground mediaby using the potential difference measured on the water surface. Usingthese data, it is possible to obtain an underground electric resistivitystructure.

In addition, as mentioned above, for exact analysis of an undergroundstructure below a riverbed, it is required to insert the electricresistivity data of water as a limit condition during the inversion. Forthis purpose, electric conductivity of water and water depth aremeasured using the conductivity meter 130 and the side scan sonar 140mounted to the lower portion of the survey boat.

A water temperature sensor is attached to the conductivity meter 130 tomeasure water temperature together with electric conductivity of water.It is because activity of ions in water is changed according to thewater temperature, and thus the electric conductivity is also changed.

The side scan sonar 140 sends ultrasonic waves into the water and thenreceives ultrasonic waves reflected on the riverbed, thereby measuring awater depth.

The electric conductivity and depth data of water, measured as above,are added with the GPS data of the survey boat and then stored as GISdata, and then they are input as limit conditions when electricresistivity inversion operating according to a measurement position.

As mentioned above, the present invention allows rapid and accurateanalysis of an underground structure below a riverbed since water depthand electric conductivity are measured together, and then they areinserted as limit conditions for inversion.

The embodiments of the present invention are disclosed in the drawingsand the specification. However, specific terms used herein are notintended to limit the scope of the present invention defined in theappended claims but to give better explanation of the present invention.Therefore, a person having ordinary skill in the art will understandthere may be made various modifications and equivalents from theembodiments. Thus, the sincere scope of the present invention should bedefined by the technical spirit of the appended claims.

INDUSTRIAL APPLICABILITY

The system for streamer electric resistivity survey and the method foranalysis of an underground structure below a riverbed using the sameaccording to the present invention may be applied to analysis of anunderground structure such as a border of bedrock below a riverbed.

1-7. (canceled)
 8. A method for analysis of an underground below ariverbed using the measured data at an operating device that includes amemory, a data operator, an underground structure analyzer, and acontroller, the method comprising: (a) the data operator readingposition data of a survey ship and position data of a tail of a streamercable from the memory to generate precise measurement position data; (b)the underground structure analyzer conducting two-dimensional inversionto the measured electric resistivity data according to the precisemeasurement position data; and (c) the underground structure analyzerimaging an underground structure below a riverbed using the inversionresult, wherein, in the step (b), a water depth and an electricconductivity of water are input as limit conditions.
 9. A method foranalysis of an underground below a riverbed using the measured data atan operating device that includes a memory, a data operator, anunderground structure analyzer, and a controller, the method comprising:(a) the data operator reading electric conductivity data of water andposition data of a survey ship from the memory to generate an electricconductivity map of water according to position; (b) the data operatorreading water depth data and the position data of the survey ship fromthe memory to generate a water depth map according to position; (c) thedata operator reading the position data of the survey ship and positiondata of a tail of a streamer cable to generate precise measurementposition data; (d) the underground structure analyzer conductingtwo-dimensional inversion to the measured electric resistivity dataaccording to the precise measurement position data; and (e) theunderground structure analyzer imaging an underground structure below ariverbed using the inversion result, wherein, in the step (d), theelectric conductivity data of water according to positioned obtained inthe step (a) and the water depth data according to position obtained inthe step (b) are input as limit conditions.